In recent years, the request for the introduction of a next-generation 40 Gbit/s optical transmission system has increased. Furthermore, it has been requested that the 40 Gbit/s optical transmission system should have the same transmission range and frequency use efficiency as those of a 10 Gbit/s system. To realize this, research and development for RZ-DPSK (Differential Phase-Shift Keying) modulation or CSRZ-DPSK modulation become active. The RZ-DPSK or CSRZ-DPSK modulation is a modulation method that is excellent in an optical signal to noise ratio (OSNR) bearing force and a nonlinear bearing force as compared to an NRZ (Non Return to Zero) modulation method that has been applied to a conventional 10 Gbit/s or less system. In addition to the modulation methods, research and development for a phase modulation method also become active. The phase modulation method includes RZ-DQPSK (Differential Quadrature Phase-Shift Keying) modulation or (CS) RZ-DQPSK modulation that has the special feature of a narrow spectrum (high frequency use efficiency) (for example, see G. Charlet et al., “Non-liner Interactions between 10 Gb/s NRZ Channels and 40 Gb/s Channels with RZ-DQPSK or PSBT Format over Low-Dispersion Fiber”, ECOC 2006 Mo.3.2.6.).
FIG. 11 is a diagram illustrating a configuration example of a light transmitting device 10 and a light receiving device 30 that employ a 43 Gbit/s RZ-DPSK or CSRZ-DPSK modulation method. FIG. 12 is a diagram illustrating the states of a light intensity and an optical phase when transmitting and receiving an optical signal modulated in a RZ-DPSK or CSRZ-DPSK modulation method.
In FIG. 11, the light transmitting device 10 transmits an optical signal modulated in a 43 Gbit/s RZ-DPSK or CSRZ-DPSK modulation method. For example, the light transmitting device 10 includes a transmitted data processing unit 11, a CW (Continuous Wave) light source 12, a phase modulator 13, and an RZ-pulsed intensity modulator 14.
Specifically, the transmitted data processing unit 11 has a function that acts as a framer that frames data to be input, a function that acts as an FEC (Forward Error Correction) encoder that provides an error correction code, and a function that acts as a DPSK precoder that performs an encoding process in which difference information between a code before one bit and a current code is reflected.
The phase modulator 13 modulates the phase of continuous light output from the CW light source 12 in accordance with encoded data output from the transmitted data processing unit 11, and outputs an optical signal of which the light intensity is constant and the binary optical phase carries on information, that is to say, an optical signal that is modulated in a DPSK method (see the lower stage of FIG. 12).
The intensity modulator 14 RZ-pulses the optical signal output from the phase modulator 13 (see the upper stage of FIG. 12). Particularly, an RZ-DPSK signal indicates an optical signal RZ-pulsed by using a clock driving signal that has the same frequency (43 GHz) as the bit rate of data and an amplitude obtained by multiplying an extinction voltage (Vπ) by one. Moreover, a CSRZ-DPSK signal indicates an optical signal RZ-pulsed by using a clock driving signal that has a frequency (21.5 GHz) obtained by multiplying the bit rate of data by ½ and an amplitude obtained by multiplying an extinction voltage (Vπ) by two.
The light receiving device 30 is connected to the light transmitting device 10 via a transmission path 20 and an optical repeater 21, and performs received signal processing on a (CS) RZ-DPSK signal output from the light transmitting device 10 in an optical repeating transmission method. For example, the light receiving device 30 includes a delay interferometer 31, a photo-electric converting unit 32, a reproducing circuit 33, and a received data processing unit 34.
Specifically, the delay interferometer 31 is, for example, a Mach-Zehnder interferometer. The delay interferometer 31 causes a delay component of a one-bit time (23.3 ps in the configuration example of FIG. 11) to interfere (delay-interfere) in a component on which a zero-radian phase control is performed, with respect to the (CS) RZ-DPSK signal transmitted through the transmission path 20, and outputs the interference result as two outputs. In the Mach-Zehnder interferometer, a split wave guide is formed to be longer than another split wave guide by a propagation length corresponding to a one-bit time and an electrode is formed to control the phase of an optical signal propagating the other split wave guide.
The photo-electric converting unit 32 is a dual pin photodiode that performs differential photoelectric conversion detection (balanced detection) by respectively receiving outputs from the delay interferometer 31. In addition, a received signal detected by the photo-electric converting unit 32 is appropriately amplified by an amplifier.
The reproducing circuit 33 extracts a data signal and a clock signal from the received signal detected by the photo-electric converting unit 32 in a differential photoelectric conversion method. The received data processing unit 34 performs signal processing such as error correction based on the data signal and clock signal extracted by the reproducing circuit 33.
FIG. 13 is a diagram illustrating a configuration example of a light transmitting device 40 and a light receiving device 60 that employ a 43 Gbit/s RZ-DQPSK or CSRZ-DQPSK modulation method. FIG. 14 is a diagram illustrating the states of a light intensity and an optical phase when an RZ-DQPSK or CSRZ-DQPSK modulated optical signal is transmitted and received.
For example, in FIG. 13, the light transmitting device 40 includes a transmitted data processing unit 41, a 1:2 separating unit (DMUX) 42, a CW light source 43, a π/2 phase shifter 44, two phase modulators 45A and 45B, and an RZ-pulsed intensity modulator 46.
Specifically, the transmitted data processing unit 41 includes functions as a framer and an FEC encoder and a function as a DQPSK precoder that performs an encoding process in which difference, information between a code before one bit and a current code is reflected, similarly to the transmitted data processing unit 11 illustrated in FIG. 11.
The 1:2 separating unit 42 divides 43 Gbit/s encoded data output from the transmitted data processing unit 41 into 21.5 Gbit/s two-series encoded data #1 and #2. The CW light source 43 outputs continuous light. The outputted continuous light is divided into two light beams. One light beam is input into the phase modulator 45A and the other light beam is input into the phase modulator 45B via the π/2 phase shifter 44.
The phase modulator 45A modulates the continuous light output from the CW light source 43 by using one-series encoded data #1 divided by the 1:2 separating unit 42 and outputs an optical signal in which information is carried on a binary optical phase (0 radian or π radians). Moreover, the phase modulator 45B receives light that is obtained by phase-shifting the continuous light output from the CW light source 43 by π/2 in the π/2 phase shifter 44, modulates the received light by using the other-series encoded data #2 separated by the 1:2 separating unit 42, and outputs an optical signal in which information is carried on a binary optical phase (π/2 radians or 3π/2 radians).
The light modulated by the phase modulators 45A and 45B is multiplexed and then is output to the subsequent-stage RZ-pulsed intensity modulator 46. In other words, modulated light output from the phase modulators 45A and 45B is multiplexed, and thus an optical signal (see the lower stage of FIG. 14) of which the light intensity is constant and information is carried on a four-value optical phase, that is to say, a DQPSK-modulated optical signal is sent to the RZ-pulsed intensity modulator 46.
The intensity modulator 46 RZ-pulses the DQPSK-modulated optical signal output from the phase modulators 45A and 45B similarly to the intensity modulator 14 illustrated in FIG. 11. Particularly, an RZ-DQPSK signal indicates an optical signal RZ-pulsed by using a clock driving signal that has the same frequency (21.5 GHz) as the bit rate of data #1 and #2 and an amplitude obtained by multiplying an extinction voltage (Vπ) by one. Moreover, a CSRZ-DQPSK signal indicates an optical signal RZ-pulsed by using a clock driving signal that has a frequency (10.75 GHz) obtained by multiplying the bit rate of data #1 and #2 by ½ and an amplitude obtained by multiplying an extinction voltage (Vπ) by two.
The light receiving device 60 is connected to the light transmitting device 40 via a transmission path 50 and an optical repeater 51, and performs received signal processing on the (CS) RZ-DQPSK signal output from the light transmitting device 40 in an optical repeating transmission method. For example, the light receiving device 60 includes a splitting unit 61 that splits the received optical signal into two. The light receiving device 60 further includes delay interferometers 62A and 62B, photo-electric converting units 63A and 63B, and reproducing circuits 64A and 64B on optical signal paths on which the split optical signals are propagated. Furthermore, the light receiving device 60 includes a 2:1 multiplexing unit (MUX) 65 that multiplexes the data signal reproduced by the reproducing circuits 64A and 64B and a received data processing unit 66.
Specifically, the delay interferometers 62A and 62B respectively receive the optical signals that are obtained by splitting the (CS) RZ-DQPSK signal transmitted through the transmission path 50 and the optical repeater 51 into two by using the splitting unit 61. The delay interferometer 62A causes the delay component of a one-bit time (46.5 ps in the configuration example of FIG. 13) to interfere (delay interfere) in a component on which the phase control of π/4 radians is performed, and outputs the interference result as two outputs.
Moreover, the delay interferometer 62B causes the delay component of the one-bit time to interfere (delay interfere) in a component (the phase deviates by π/2 radians from the same component of the delay interferometer 62A) on which the phase control of −π/4 radians is performed, and outputs the interference result as two outputs. In this case, each of the delay interferometers 62A and 62B is configured of a Mach-Zehnder interferometer, and is configured of a dual pin photodiode that performs differential photoelectric conversion detection by receiving light. In addition, the received signals detected by the photo-electric converting units 63A and 63B are appropriately amplified by an amplifier.
The reproducing circuit 64A reproduces an in-phase component I for the clock signal and data signal from the received signal detected by the photo-electric converting unit 63A in a differential photoelectric conversion method. Moreover, the reproducing circuit 64B reproduces a quadrature-phase component Q for the clock signal and data signal from the received signal detected by the photo-electric converting unit 63B in a differential photoelectric conversion method.
The 2:1 multiplexing unit 65 receives the in-phase component I and the quadrature-phase component Q that are respectively output from the reproducing circuits 64A and 64B, and converts them into a 43 Gbit/s data signal before DQPSK modulation. The received data processing unit 66 performs signal processing such as error correction based on the data signal output from the 2:1 multiplexing unit 65.
As described above, a market demands the realization of a wavelength division multiplexing transmission system that operates based on a multiple signal obtained by mixing a 40 Gbit/s phase modulated signal (a signal modulated by (CS) RZ-DQPSK modulation method or (CS) RZ-DPSK modulation method) and a conventional 10(2.5) Gbit/s intensity modulated signal (a signal modulated by an NRZ method).
However, when the wavelength division multiplexing transmission system is configured, the 40 Gbit/s phase modulated signal receives an optical phase shift caused by cross phase modulation (XPM) from the 10(2.5) Gbit/s intensity modulated signal. Therefore, there is a problem in that a waveform is conspicuously degraded and long distance transmission becomes difficult.
In other words, it extremely becomes an important problem to prevent the waveform degradation (XPM degradation) of a phase modulated signal even if the phase modulated signal and the intensity modulated signal are wavelength-multiplexed.